The invention relates generally to an apparatus for use in catalytic distillation processes, and more particularly to column internals for catalytic distillation processes. The present invention is particularly suited for column internals utilizing catalyst beds and trays with multiple stage sections.
Catalytic distillation processes are processes in which at least one chemical reaction is carried out in the presence of at least one solid catalyst. The chemical reaction occurs simultaneously with distillation of a reaction mixture obtained within one vessel. Several types of apparatus have been described for use in a variety of catalytic distillation processes.
The key part of developing catalytic distillation technology is the design of internal elements or tower internals for the catalytic distillation columns. The functions of column internals are to carry out both the catalytic reaction and mass transfer simultaneously. In the past fifteen years, numerous patents have been awarded for the design of the new internals. They can be classified as follows:
1. Cloth Belt (U.S. Pat. No. 4,215,011). The catalyst is sealed within a cloth belt. The belt is then wrapped in an open mesh knitted non-reactive wire. Liquid can penetrate into and flow out from the catalyst through the cloth. The liquid wetted surface of the cloth belt provides the vapor-liquid interfacial area for mass transfer.
The mass transfer efficiency for this type of catalyst unit is very low because of the low interfacial area. The effectiveness of the catalyst inside the belt may change from location to location because of the different liquid residence time within the cloth. This reduces the overall system efficiency.
2. Catalyst Container Held on a Tray (U.S. Pat. Nos. 4,536,373, 4,439,350, 5,447,609, 5,449,501, 5,776,320, 5,792,428, 5,888,355 and 6,045,762). One type of this catalytic distillation unit consists of a normal distillation tray and a parallel array of rectangular tubes (troughs) filled with a catalyst. The tubes (troughs) are constructed with metal screen and closed at both ends. The parallel tubes (troughs) may be supported above a distillation tray and should be located within a froth zone when the column is in operation. The liquid reaction mixture flows across the tray either parallel to or at right angles to the tubes (troughs).
Another type of this unit consists of a conventional distillation tray and closed porous containers containing catalyst. The containers are installed above or on the distillation tray in the tower. At least one tray may be combined with a packed catalyst bed. Another type of unit includes catalyst containing reaction zones in the downcomer in which liquid reaction mixture rises through a catalyst bed before exiting downward from an opening at the top of the reaction zone.
The structure of these units is usually quite complex, thus leading to a greater cost in the overall system. The catalyst quantity that can be loaded is also limited by both volume and spacing. It is not suitable if the catalytic reaction step is slow because the reaction rate may not be matched with the mass transfer rate and will lower the overall efficiency of the system.
3. Packings Random packings (U.S. Pat. Nos. 4,443,559, 5,057,468, 5,275,790, 5,262,012, and 5,189,001), e.g. Raschig rings, can be made of a polymer catalyst such as ion exchange resins. The random packing in the column acts as both a catalyst and an aid to mass transfer. The random packing can also be made from rigid containers having a volume substantially smaller than the volume of a reactive distillation column.
The catalyst can be loaded into the container. Openings in the containers are provided to allow vapor and liquid passage into and out of the containers. The surfaces of the containers provide the necessary facets for improving vapor-liquid contact for the distillation. The rigidity of the containers provides for the spacing of the structures and the necessary free space for the distillation.
Another type of this unit consists of a catalyst component and a resilient component intimately associated therewith. The resilient component has at least about 70% volume open space and is present with the catalyst component in an amount such that the catalyst distillation structure consists of at least 10% volume open space.
Structured Packings (U.S. Pat. Nos. 5,073,236, 5,235,102, and 5,348,710) are constructed using corrugated screen envelopes with catalyst filled inside. Each envelope consists of two layers of crimped screen, roughly 30 centimeters (1 ft.) square, which are sealed at the edges. The envelopes are stacked and bound to make “bricks”, which in turn are stacked to fill the column. The packing can also be fabricated in rigid, cellular monoliths or in a rigid, cellular monolith coated with a catalytically active material.
Liquid reaction mixture holdup or the time the liquid reaction mixture is retained within this type of column internals is low. Therefore, it is not suitable for a slow reaction. Like cloth belts, the effectiveness of the catalyst at the surface may be different from that catalyst at the inside because of the different liquid residence time within the packing. Additionally, the complex nature of the screens and packing leads to an increased cost of the internals and system.
4. Tray Plus Fixed Bed of Catalyst (U.S. Pat. Nos. 5,130,102, 5,368,691, 5,013,407, 5,026,459 and 6,441,252). In these catalytic distillation units, the reactive zone consists of alternate beds of catalyst and catalyst-free distillation zones. Passageways may be provided for a vapor in the fixed bed. Distillation zones contain conventional distillation trays and liquid distribution plates.
The structures of these units typically are complex and relatively high in cost. The liquid flow pattern may be far from plug flow or single direction flow. Therefore, their efficiency could be low.
5. Conventional Distillation Tray with Catalyst Placed in the Downcomer (U.S. Pat. Nos. 3,579,309, 5,277,847, 3,629,478, and 3,634,535). In these units, downcomers attached to conventional trays are filled with catalyst, which serve as a reaction zone, and the trays act as a distillation zone.
In these units, a portion of the catalyst may not be exposed to liquid reaction mixture, thus reducing its activity. Also, loading is limited by the downcomer size. It is not suitable for slow reaction processes. The catalyst bed may pose a restriction to the liquid reaction mixture flow in the downcomer which in turn may cause a backup of liquid reaction mixture onto the tray, thereby reducing distillation efficiency.
6. Fixed Catalyst Bed with a Distribution Element (U.S. Pat. No. 5,523,062). In this arrangement, a corrugated plate having openings only at the peaks or valleys of the corrugation is used as a distribution element for the fixed catalyst bed. The plate and fixed bed are installed alternatively. The plate makes little or no contribution to mass transfer.
The liquid reaction mixture distribution to the lower catalyst bed can deteriorate if the plate is not properly installed. Radial mixing of liquid reaction mixture in the column is minimal. All of these beds may cause great uncertainty in the operation of such a catalytic distillation unit. For the catalyst part, the same disadvantages exist as described for packings, shown above in (3). Steps in a complex process may be conducted as discreet catalytic distillation processes in a sequence of units (U.S. Pat. Nos. 6,294,684, 6,407,300 and 6,767,517).
It will be apparent that each of the different types of units has its advantages and disadvantages. However, none the above-described systems provides structures that may easily be adapted to couple a specific reaction with a desired distillation, i.e. for any given catalytic reaction there is a corresponding distillation rate. There is a need for both improved distribution of the liquid reaction mixture and improved contact of said liquid reaction mixture with the catalyst during catalytic distillation.